Possible involvement of μ 1-opioid receptors in the fentanyl- or morphine-induced antinociception at supraspinal and spinal sites

Abstract

Fentanyl has been shown to be a potent analgesic with a lower propensity to produce tolerance and physical dependence in the clinical setting. The present study was designed to investigate the mechanisms of fentanyl- or morphine-induced antinociception at both supraspinal and spinal sites. In the mouse tail-flick test, the antinociceptive effects induced by both fentanyl and morphine were blocked by either the μ 1-opioid receptor antagonist naloxonazine or the μ 1/μ 2-opioid receptor antagonist β-funaltrexamine (β-FNA) after s.c., i.c.v. or i.t. injection. In contrast, both fentanyl and morphine given i.c.v. or i.t. failed to produce antinociception in μ 1-deficient CXBK mice. These findings indicate that like morphine, the antinociception induced by fentanyl may be mediated predominantly through μ 1-opioid receptors at both supraspinal and spinal sites in mice. We also determined the ED 50 values for s.c.-, i.c.v.- and i.t.-administered fentanyl- or morphine-induced antinociception in mice. The ED 50 values for s.c.-, i.c.v.- and i.t.-administered fentanyl-induced antinociception were 73.7, 18.5 and 1.2-fold lower than that of morphine, respectively. The present data clearly suggest the usefulness of peripheral treatment with fentanyl for the control of pain.